A167768 First of 4 or more consecutive integers with equal values of phi(phi(n)).

1, 7, 31, 32, 2694, 131071, 50802031105

Offset

1,2

Comments

Next term > 1266046940 (last term of A167767 b-file). - Michel Marcus, Jun 23 2013

No other terms < 1.32*10^12. - Jud McCranie, Jul 19 2017

Links

Table of n, a(n) for n=1..7.

Example

p2(1) = p2(2) = p2(3) = p2(4) = 1, p2(7) = p2(8) = p2(9) = p2(10) = 2.

Mathematica

a[n_] := EulerPhi[EulerPhi[n]]; Select[Range[10000], a[#] == a[# + 1] && a[# + 1] == a[# + 2] && a[# + 2] == a[# + 3] &] (* G. C. Greubel, Jun 23 2016 *)

Prog

(PARI) pp(n) = eulerphi(eulerphi(n))
isA167768(n) = pp(n)==pp(n+1) && pp(n+1)==pp(n+2) && pp(n+2)==pp(n+3) \\ Michael B. Porter, Nov 24 2009

Crossrefs

Cf. A167767 (of which this list is a subset), A167766.

Sequence in context: A393189 A394551 A184121 * A103630 A205711 A040064

Adjacent sequences: A167765 A167766 A167767 * A167769 A167770 A167771

Keyword

more,nonn,hard

Author

Fred Schneider, Nov 11 2009

Extensions

Edited by N. J. A. Sloane, Nov 12 2009

a(7) added by Jud McCranie, Jul 17 2017

Status

approved